There is a growing demand for the photovoltaic devices as the importance thereof has been recognized more and more as a clean energy source. The fields where the demand for the photovoltaic devices has increased are various and range from power energy sources of large equipments to small-sized power supplies for the precision electronic devices. In this specification, the term “photovoltaic devices” indicates both a photovoltaic device module (hereinafter sometimes referred to simply as a “module”) and a photovoltaic cell (hereinafter sometimes referred to simply as a “cell”).
Wide application of the photovoltaic devices in various fields is expected to meet various inconveniences on the part of the photovoltaic devices user unless the characteristics or especially the current-voltage characteristics of the photovoltaic devices are accurately measured.
For that reason, a pseudo-sunlight radiator (hereinafter after referred to as a solar simulator) to measure the current-voltage characteristics of the photovoltaic devices has conventionally been proposed and used in practical applications. Nevertheless, there still exist problems to be solved.
Specifically, the solar simulator is used to radiate the artificial light (pseudo-sunlight) of equalized irradiance on the test plane of photovoltaic devices in panel form and thus measure the current-voltage characteristics of the photovoltaic devices. Though the lamp is assumed to be a form considered as a spot or a line, it is very difficult to radiate the light with the equalized irradiance to the whole test plane (or whole area) of the photovoltaic devices.
In the prior art, efforts have been made to improve the problem of light emitted with equalized irradiance mentioned above. However, most of the methods thus far developed are no more than an improvement of the optical system or the light-emitting (lighting) circuit system of the light source.
Improvements in the optical system or the light-emitting circuit system of the light source of the solar simulator have been proposed in Patent Documents 1 to 3. In spite of the improved light source, it has been difficult to improve the locative unevenness of irradiance on the test plane of the object to be measured by less than ±2 to ±3%.
In the ordinary measurement of the photovoltaic device module, the irradiance to the effective plane section is set at 1000 W/m2. FIG. 1 is a diagram showing the I-V curve indicating the current-voltage characteristics of the object to be measured (photovoltaic device module) irradiated by such irradiance comes to graphically the form illustrated in FIG. 1. In this diagram, I designates the current, and V the voltage.
When the locative unevenness of irradiance on the test plane is given out by the irradiated light of the solar simulator, the I-V curve changes as shown on FIG. 2., and therefore the current-voltage characteristics of the photovoltaic devices cannot be measured correctly.
Incidentally, the FIG. 2 shows a comparison of the I-V curve in the five types of a 96 serial module that is a photovoltaic device module with 96 photovoltaic cells connected in series (herein after it is referred to as the “96 serial module”). The five types of the 96 serial module mean a type of the 96 serial module having 0 cell shaded by 10% (i.e. the state without the locative unevenness of irradiance) and four types of the 96 serial module having 1 cell, 2 cells, 3 cells and 10 cells, shaded by 10% respectively (i.e. the state with unevenness of irradiance of different degrees made artificially).
Curve (a) indicates the ideal state in which the locative unevenness of irradiance on the test plane is less than ±1.0%.
Curves (b) to (e) is I-V curves of the 96 serial modules in which the locative unevenness of irradiance of different degree is made artificially using 1, 2, 3 and 10 object cells shaded by 10%.
As shown in the diagram of FIG. 2, when a change in the I-V curve due to the locative unevenness of irradiance is given out, the form factor FF=Pmax/Isc×Voc (where Pmax is the maximum output of the module, Isc the shorting current, and Voc the open-circuit voltage) changes even for the photovoltaic cells of the same configuration. This point is explained with reference to FIG. 3.
The FIG. 3 shows the changes of the form factor FF and the Pmax value according to the quantity of cell shaded by 10% in the above mentioned 96 serial module.
The left side of the ordinate is the FF (form factor in %) value and the right side of the ordinate the Pmax (maximum output, W) value.
The abscissa shows the quantity of cell shaded by 10%, used for making the locative unevenness of irradiance artificially.
The FF value and the Pmax value in accordance with the increase of the quantity of cell are respectively plotted and drawn by a line.
For the module in the ideal state that the locative unevenness of irradiance is less than ±1.0% (i.e. the state that the quantity of shaded cell is 0.), the value Pmax for is 159.1 (W) and the FF value is 70.7%.
On the contrary, for each module having 1 to 4, 6, 8, 10 and 12 shaded cells used for producing artificial locative unevenness of irradiance, the Pmax value is varied from 154.2 to 152.0 (W) and the FF value is also varied between 68.8% and 74.1%.
FIG. 4 shows the changes of the average value of the irradiance in accordance with the varying of the quantity of cells of the 96 serial module in the test module as illustrated in the FIG. 3.
Curve A1 indicates the irradiance ratio between a mean value of the irradiances measured at five points (four points at the four corners of the cell and one point at the center thereof) on the test plane of the module and a mean value of the irradiances of all the cells forming the test plane of the test module in a general measurement method.
Curve B1 indicates the ratio between the Pmax value for zero cell shaded by 10% (no uneven irradiance) and the Pmax value of each module using 1 to 12 cells shaded by 10%.
According to the Curve A1 and the Curve B1, it can be seen that for the module in the ideal state (i.e. with zero shaded cell) without locative unevenness of irradiance (less than ±1.0%), the disparity is very small between the Pmax value ratio and the irradiance ratio. On the other hand, for each test module making artificial uneven irradiance, the disparity between the numerical values of the two curves A1 and B1 is large.
The above describing shows that, it is difficult to measure the correct current-voltage characteristics when there is any locative unevenness on the test plane of the module.
[Patent Document 1] Japanese Patent Provisional Publication No. 8-235903
[Patent Document 2] Japanese Patent Provisional Publication No. 9-306201
[Patent Document 3] Japanese Patent Publication No. 6-105280